To test, in vivo, the hypothesis that exosomes from multipotent mesenchymal stromal cells (MSCs) mediate microRNA 133b (miR-133b) transfer which promotes neurological recovery from stroke, we used knockin and knockdown technologies to upregulate or downregulate the miR-133b level in MSCs (miR-133b⁺MSCs or miR-133b⁻MSCs) and their corresponding exosomes, respectively. Rats were subjected to middle cerebral artery occlusion (MCAo) and were treated with naïve MSCs, miR-133b⁺MSCs, or miR-133b⁻MSC at 1 day after MCAo. Compared with controls, rats receiving naïve MSC treatment significantly improved functional recovery and exhibited increased axonal plasticity and neurite remodeling in the ischemic boundary zone (IBZ) at day 14 after MCAo. The outcomes were significantly enhanced with miR-133b⁺MSC treatment, and were significantly decreased with miR-133b⁻MSC treatment, compared to naïve MSC treatment. The miR-133b level in exosomes collected from the cerebral spinal fluid was significantly increased after miR-133b⁺MSC treatment, and was significantly decreased after miR-133b⁻MSC treatment at day 14 after MCAo, compared to naïve MSC treatment. Tagging exosomes with green fluorescent protein demonstrated that exosomes-enriched extracellular particles were released from MSCs and transferred to adjacent astrocytes and neurons. The expression of selective targets for miR-133b, connective tissue growth factor and ras homolog gene family member A, was significantly decreased in the IBZ after miR-133b⁺MSC treatment, while their expression remained at similar elevated levels after miR-133b⁻MSC treatment, compared to naïve MSC treatment. Collectively, our data suggest that exosomes from MSCs mediate the miR-133b transfer to astrocytes and neurons, which regulate gene expression, subsequently benefit neurite remodeling and functional recovery after stroke. STEM Cells 2013;31:2737–2746